Coil assembly for an electric magnet



Dec. 15, 1964 T, R. BROGAN ETAL 3,161,807

COIL. ASSEMBLY FOR AN ELECTRIC MAGNET Filed March 1. 1961 5 Sheets-Sheet1 THOMAS R. BROGAN JOHN W. LOTHROP INVENTORS Wm 17. PM

ATTORNEYS 1964 T. R. BROGAN ETAL 3,161,807

con. ASSEMBLY FOR AN ELECTRIC MAGNET Filed March 1. 1961 3 Sheets-Sheet2 Q THOMAS R. BROGAN JOHN W. LOTHROP INVENTORS AT TORNEYS Dec. 15, 1964T. R. BROGAN ETAL COIL. ASSEMBLY FOR AN ELECTRIC MAGNET Filed March 1.1961 3 Sheets-Sheet 3 FIELD DENSITY (B) IRON CORE MAGNE T AIR CORE MAGNET MAGNETIZING FORCE (H) THOMAS R. BROGAN JOHN W. LOTHROP INVENTORSBYflMLwDF M W ATTORNEYS United States Patent O" 3,161,807 COIL ASSEMBLYFOR AN ELECTRIC MAGNET Thomas R. Brogan, Arlington, and John W. Lothrop,Essex, Mass, assignors to Avco Corporation, Cincirn nati, Ohio, acorporation of Delaware Filed Mar. 1, 1961, Ser. No. 92,537 8 Claims.(Cl. 317-158) The present invention relates to electric magnets and,more particularly, to electric magnets for providing substantiallyuniform high field strengths in a region perpendicular to the commonaxis of the magnet winding and extending through the magnet.

Some magnets, such as are used to furnish the mag netic field in amagnetohydrodynamic (hereinafter referred to as Ml-ID) generator, oftenoperate at low voltages but require very large currents of the order ofthousands of amperes. Such large currents make it necessary that theconductors forming the turns of these windings have such large crosssections that they cannot be bent into the required shape.

The general object of the present invention is to provide an electricmagnet, the winding conductor being of large cross section and formed toprovide a passage through the magnet perpendicular to the common axis ofthe winding to provide a substantially uniform and high strength fieldin the passage.

Another object of the present invention is the provision of a high fieldstrength magnet comprised of individual turns formed from sheets ofinsulated conductive material laid on top of each other whereinoppositely disposed portions of the center turns are bent to the side tocreate a region extending through the magnet and parallel to the turnswhereby a magnetic field is provided that is substantially constant in adirection transverse of the turns.

The novel features that are considered characteristic of the presentinvention are set forth in the appended claims. The invention itself,however, both as to its organization and method of operation, togetherwith additional objects and advantages thereof, will best be understoodfrom the following description of a specific embodiment when readlinconjunction with the accompanying drawings, in which:

FIGURE 1 is a diagrammatic illustration of an MHD generator;

FIGURE 2 is a side view partly in section of a magnet constlucted inaccordance with the present invention;

FIGURE 3 is a cross sectional view taken along line 3-3; t 7

FIGURE 4 and FIGURE 5 are perspective views of metal blanks from whichthe winding may be formed;

FIGURE6 is a fragmentary side view illustrating the mannerin which theturns are electrically connected one to another; and

FIGURE 7 is a graphic illustration of the BI-I curve from which itexhausts, as indicated by the arrow at 3.

The pressure at the exit of the duct is lower than at its inlet; and forthis reason the ,plasma moves at high 3,161,807 Patented Dec. 15, 1964velocity through the duct, as indicated by the arrow at 4. By properlychoosing the pressure differential and shape of the duct, the plasma canbe made to move through the duct at substantially constant velocity,which is desirable, although not necessary, to the operation of thegenerator. Surrounding the exterior of the duct is a continuouselectrical conductor in the form of a coil 5 to which a unidirectionalelectrical current may be supplied from any conventional source or fromthe generator itself. Flow of electrical current through the coilestablishes a magnetic flux through the duct perpendicular to thedirection of plasma flow and the plane of the paper.

Within the duct are provided opposed electrodes 6 and 7. Theseelectrodes may extend along theinterior of the duct parallel to thedirection of plasma movement and may be positioned opposite one anotheron an axis perpendicular to both the direction of plasma movement andthe magnetic flux. High velocity movement of the electrically conductiveplasma through the magnetic field induces a unidirectional electromotiveforce between the electrodes, as indicated by the arrows at 8.

The electrodes 6 and 7 are connected by conductors 11 and 12 to a load13 through which electrical current flows under the influence of theelectromotive force induced between the electrodes.

From the foregoing description, it will be immediately recognized thatan MHD generator of the type described employs. a stationary magneticfield and unidirectional gas flow. As a result, such a generator isinherently a source of direct current. If alternating currentis desired,specially designed generators or auxiliary equipment must be provided toinvert the direct current to alternating current. v

The electric magnet shown in FIGURE 2 and FIG- URE 3 includes a winding21 comprised of end turns indicated generally by the numerals 22 and 23and center turns indicated by the numeral 24. The end turns 22 and 23are essentially alike as are the center turns 24 except as will behereinafter pointed out. The end turns 22 and 23 surroundrespectivelyiron pole pieces 25 and 26. Each pole piece 25 and 26 isattached as by bolts 27 and 28 to respectively end pieces 29 and 30which extend to about oppositely disposed edges of the turns as bestshown in FIGURE 3. The center portion of the winding intermediate itsends rests upon and is supported by the bottom end piece 30 and the topend piece 29, in turn, rests upon the center portion of the winding.Insulation 31 is provided between the turns of the winding and thewind-ing and the end pieces to prevent shorting of the winding. Endpiece 30 is mounted on a suitable base 32, such as, for example, I beamsas shown. I The bolts 28 serve the additional function of attaching thebottom pole piece 26 to the base 32, and the bolts 27 similarly servethe additional function of attaching the top pole piece 25 tocrossmembers 33, such as, for-example, I beams as shown. The crossmembers 33 and the base 32 which compress the winding 21, are held infixed relation one with another by rods 34. Rods 34 and bolts 27 and 28function to provide a rigid structure to prevent displacement of thevarious parts of the magnet due to the'force exerted thereon whencurrent is first supplied to the magnet and when. the magnet is shutdown. Suitable means such as, for example, I beams 41 and members 42,prevent displacement of the bent portions 44 (more fully describedhereinafter) of the center turns 24- that are bent aside.

Members 42 are T-shaped and maybe formed of a nonconductive material orinsulated from the turns as by insulation 31. By reason of theireffective abutment with the end pieces 29 and 3t and the innermostsurface 43 of the bent portions 44 of the center turns that are bentaside, members 42 in addition to supporting the overhang of the winding21 prevent displacement of the said bent portions 44 inwardly toward thecommon axis of the turns. I beams 41 connected by stress members 45 areessential to prevent displacement of the bent portions 44 of the centerturns in the opposite direction (away from the common axis) which wouldotherwise occur during operation of the magnet if I beams 41 were notpresent. It is essential that the bent portions 44 be maintained in afixed position for the reason that operation of the magnet will tend tocause displacement thereof and consequent failure of one or more of theconnections between the turns due to the stresses on these connectionswhich will result from displacement of the bent portions of the centerturns.

The magnet having now been generally described, the winding and itsconstruction will now be described in detail.

FIGURE 4 and FIGURE illustrate respectively by way of example thegeneral configuration of the end turns and the center turns. As shown inFIGURE 4 and FIG- URE 5, the turns of the winding are made from blanks51 and 52 in the form of a heavy fiat metal plate, preferably copper,with respectively openings 53 and 54. Each blank 51 and 52 is thus acontinuous turn surrounding its respective opening. These blanks may be.formed in any desired way as by stamping, shearing, cutting, orpunching them from solid fiat metal sheets.

Each end turn in the finished winding is fiat and extends around thespace formed by openings 53 and in which are located the pole pieces.These turns are electrically connected one to another, such as, forexample, in the manner hereinafter to be described. At one point on eachblank a slot 55 (FIGURE 6) is cut through the blank to change it from aclosed blank or turn to an open turn with two ends 56 and 57. Theadjacent turns are also slotted but in a slightly different displacedposition so that each turn may be connected to the opposite end of anadjacent turn. With reference now to FIGURE 4, the first or bottom turnmay, for example, be cut at lines 58 and 59. The next succeeding turn iscut at lines 60 and 61. The cuts in the succeeding turns are similarlyprogressively displaced such that when the turns are assembled a portionindicated by way of example in FIG- URF. 4 by the numeral 62 willcomprise the left-hand end 56 of the second turn and will extend overthis same portion which comprises the right-hand end 57 of the firstturn as shown in FIGURE 6. Insulation 31 such as Mylar having athickness of about .010 inch is provided to insulate the turns from thepole pieces or base and from each other with the exception of the area63 where the left-hand end 56 of each turn projects over the righthandend 57 of the preceding turn as shown in FIG- URE 6.

Inspection of FIGURE 6 will show that the left end 56 of each turnextends across the slot 55 resulting from conversion from the blank froma closed to an open turn and over a small portion of the opposite orright end 57 of the preceding turn. An electrically conductive spacer 64equal in thickness to the thickness of the insulation 31 is insertedbetween the overlapped end portions 56 and 57 of the turns and is fusedas by welding or soldering to the aforementioned overlapping portions 55and 57 to provide a strong and low resistance connection. In thismanner, and as illustrated in FIGURE 6, the turns are insulated one fromanother and serially connected to form the winding. A power source (notshown) may be connected to the free ends of the outermost end turns.

To provide a region extending through the winding perpendicular tothecommon axis thereof, or stating it another way. to provide accessthrough the sides of the winding to the space 53 of an assembled windingthe length of the blanks 52 from which the center turns 24 are {formed(see FIGURE 5) is increased over that of the blanks 51 from which theend turns 22 and 23 are formed. When oppositely disposed portions 66 and67 of the two innermost center turns in particular, and the remainingcenter turns in general, are bent aside preferably at right angles tothe plane of the turns, such as, for example, at or adjacent lines 68and 69, passages 70 and 71 are provided through the turns at oppositeends of the winding which communicate with the interior of the winding.

If each pole piece extends into the winding through openings 53 adistance approximately equal to the total thickness of the end turns asshown in FIGURE 2, a region comprising passages 70 and 71 and openings54 will extend through the winding 21 and will be determined by thetotal thickness of the center turns and the size of the openings in thecenter turns. Thus, this region is defined by the openings 54 in thecenter turns 24, and the inner surfaces 72 and 73 of the pole pieces 25and 26.

Obviously, the pole pieces 25 and 26 and/or end pieces 29 and 30 may beomitted if desired. However, use of pole pieces as described hereinprovides the optimum design and substantial reduction of the amount ofpower required vfor operation of the magnet, particularly at very highfield strengths. Since iron saturates at about 20,000 gauss, a pure ironcore design is not desirable for very high field strength magnets and apure air core magnet will require considerably more power than an ironcore magnet. Thus, for a field strength of about 32,000 gauss a magnetincorporating pole pieces as shown and described herein will permit areduction in the power required by about 50% over that required for apure air core magnet. This permits a reduction in the power requirementsor conductor weight for a given field and hence a substantial reductionof the cost of conductor material at the expense of a small increase inthe cost of a power source of greater capacity than would otherwise berequired. As a result, a total system of minimum cost is provided.

The effect of the pole pieces is illustrated in FIGURE 7. Curve 74 isthe B-H curve for an air core magnet, while curve 75 is the B-H curvefor the same magnet with an iron core. Up to iron saturation, the ironcore magnet produces a given field for a much smaller expenditure ofpower than the air core magnet. Above saturation, the slope of the twocurves as shown in FIG- URI-I '7 is the same, the difference in fieldstrengths thus remaining the same. In effect, above saturation, the polepieces contribute a bonus field above the air core value. The value ofthis bonus field may be determined by extrapolating the iron core B-Hcurve back to zero current with the air core BH curve slope.

For a construction of the Winding in accordance with that describedhereinabove, the packing factor (ratio of conductor volume to totalvolume) can approach unity. In fact, a packing factor of 0.96 has beenachieved. Since, for a given field, the power dissipation varies as thereciprocal of the packing factor this is a most efficient design fromthe point of view of utilization of power.

At the expense of reduction of the packing factor the turns may bemaintained in spaced relationship one with another to permit coolingthereof if desired.

When a search coil consisting of many turns of fine wire on a smallbobbin is placed in the magnet, a voltage will be induced in the searchcoil which is proportional to dB/dt. By observing dB/a't on anoscilloscope when current is supplied to the magnet, a voltage vs. timecurve may be obtained which can be integrated to obtain the total fieldstrength. Moving the search coil to different locations inside themagnet will give data for field distribution curves. The search coil maybe calibrated by inserting it in a long A.C. solenoid in which the fieldcan be calculated and'measuring the coil output voltage.

A power source of 3300 -kw. was required for a magnet constructed inaccordance with the embodiment described herein and successfully reducedto practice for MHD applications. This magnet had a field with a maximumstrength of about 32,000 gauss, a field length of about 5 feet, and ahollow cross section of 14 inches by 24 inches to receive an MHDgenerator duct. At maximum power conditions the current in the magnetwas 22,000 amperes and the voltage drop was 150 volts. The winding wascomprised of 144 copper plates or turns, 52 plates of which wereutilized as the center turns. Each end turn weighed 280 pounds and eachcenter turn, all of which were of identical length before bending,weighed 430 pounds for a total copper weight of 48,200 pounds. At highfield strengths the pole pieces contributed about 10,000 gauss.

The various features and advantages of the invention are thought to beclear from the foregoing description. Various other features andadvantages not specifically enumerated will undoubtedly occur to thoseversed in the art, as likewise will many variations and modifications ofthe preferred embodiment illustrated, all of which may be achievedwithout departing from the spirit and scope of the invention as definedby the following claims:

We claim:

1. An electric magnet comprising: a conducting winding having aplurality of center turns disposed between a plurality of end turns,oppositely disposed portions comprising the entire cross section of saidcenter turns extending past the outer periphery of said end turns andbeing bent aside out of the plane of said turns to provide openings insaid winding, said end turns being coplanar with the unbent portion ofsaid center turns and located inwardly of and spaced away from the saidoppositely disposed portions of said center turns; and means to preventdisplacement of said turns when said winding is connected to anddisconnected from a source of current.

2. An electric magnet comprising: a conducting winding having aplurality of center turns disposed between a plurality of end turns,oppositely disposed portions comprising the entire cross section of saidcenter turns extending past the outer periphery of said end turns andbeing bent aside out of the plane of said turns to provide oppositelydisposed openings in said winding, said end turns being coplanar withthe unbent portion of said center turns and located inwardly of andspaced away from the said oppositely disposed portions of said centerturns; a pole piece disposed in each end of said winding; and means toprevent displacement of said turns when said winding is connected to anddisconnected from a source of current.

3. An electric magnet comprising: a conducting winding having aplurality of center turns disposed between a plurality of end turns,oppositely disposed portions comprising the entire cross section of saidcenter turns extending past the outer periphery of said end turns andbeing bent at substantially right angles to the plane of said turns toprovide oppositely disposed openings in said winding, said end turnsbeing coplanar with the unbent portion of said center turns and locatedinwardly of and spaced away from the said oppositely disposed portionsof said center turns; and means to prevent displacement of said turnswhen said winding is connected to and disconnected from a source ofcurrent.

4. An electric magnet comprising: a conducting winding having aplurality of center turns disposed between a plurality of end turns,oppositely disposed portions of said center turns extending outwardlyfrom said winding and being bent at substantially right angles to theplane of said turns to provide oppositely disposed openings in saidwinding, said end turns being coplanar with the unbent portion of saidcenter turns and located inwardly 6. of and spaced away from the saidoppositely disposed portions of said center turns; and means to preventdisplacement of said turns in a direction normal to the plane of saidturns and displacement of said oppositely disposed portions of saidcenter turns in a direction parallel to the plane of said turns whensaid winding is connected to and disconnected from a source of current.

5. An electric magnet comprising: a winding having a plurality of turnsdisposed about a common axis said turns comprising a plurality of centerturns disposed between a plurality of end turns, each turn comprising asingle piece of flat metal having one end overlapping and connected toan end of an adjoining turn, oppositely disposed portions of said centerturns extending outwardly past said end turns and being bent aside outof the plane of said turns to provide oppositely disposed openings insaid winding; means to prevent displacement of said turns in thedirection of said common axis; and means to prevent displacement of theend portions of said center turns in a direction substantially normal tosaid common axis.

6. An electric magnet comprising: a winding having a plurality of turnsdisposed about a common axis, said turns comprising a plurality ofcenter turns disposed between a plurality of end turns, each turncomprising a single piece of fiat metal having one end overlapping andconnected to an end of an adjoining turn, oppositely disposed portionsof said center turns extending outwardly past said end turns and beingbent aside out of the plane of said turns to provide oppositely disposedopenings in said winding; means for insulating said turns; a pole piecedisposed in each end of said winding; means to prevent displacement ofsaid turns in the direction of said common axis; and means to preventdisplacement of the end portions of said center turns in a directionsubstantially normal to said common axis.

7. In an electric magnet the combination comprising: a winding having aplurality of turns disposed about a common axis, said turns comprising aplurality of center turns disposed between a plurality of end turns,each turn comprising a single piece of flat metal having one endoverlapping and connected to an end of an adjoining turn, oppositelydisposed portions of said center turns extending outwardly past said endturns and being bent at substantially right angles to the plane of saidturns to provide oppositely disposed openings in said winding, said endportions being located close to the outer surface of said winding; apole piece disposed in each end of said winding; first means to preventdisplacement of said turns in the direction of said common axis; secondmeans to prevent displacement of the end portions of said center turnsin a direction substantially normal to said common axis; and means forinsulating said turns one from another and from said first and secondmeans.

8. In an electromagnet for supplying very high field strengths, thecombination comprising: a winding having a plurality of seriallyconnected turns disposed about a common axis, said winding comprising afirst and second group of end turns and a third group of center turnsdisposed between said end turns, each turn comprising a single piece offlat conductive metal severed at one point to form the ends of saidturn, each end except one end of the outermost turns overlapping andbeing connected to the opposite end of an adjoining turn to form saidserially connected turns, said connections between turns beingprogressively displaced in successive turns about said common axis, saidcenter turns having oppositely disposed portions extendin'g outwardlypast said end turns, one half of said oppositely disposed portions ofsaid center turns being bent upwardly and one half be ing bentdownwardly at substantially right angles to the plane of said turns toprovide oppositely disposed openings in said winding normal to saidcommon axis, said bent portions of said center turns being located closeto the outer surface of said end turns; means for insulating said turnsone from another and from ground; an iron pole piece disposed in eachend of said winding; first means for preventing displacement of saidturns in either direction parallel to said common axis; and second meansfor preventing displacement of said bent portions of said center turnsin either direction perpendicular to said common axis.

References Cited in the file of this patent UNITED STATES PATENTSBehlmer Aug. 8, Woodfolk Nov. 18, Sommerville Apr. 10, Freas May 1,

1. AN ELECTRIC MAGNET COMPRISING: A CONDUCTING WINDING HAVING APLURALITY OF CENTER TURNS DISPOSED BETWEEN A PLURALITY OF END TURNS,OPPOSITELY DISPOSED PORTIONS COMPRISING THE ENTIRE CROSS SECTION OF SAIDCENTER TURNS EXTENDING PAST THE OUTER PERIPHERY OF SAID END TURNS ANDBEING BENT ASIDE OUT OF THE PLANE OF SAID TURNS TO PROVIDE OPENINGS INSAID WINDING, SAID END TURNS BEING COPLANAR WITH THE UNBENT PORTION OFSAID CENTER TURNS AND LOCATED INWARDLY OF AND SPACED AWAY FROM THE SAIDOPPOSITELY DISPOSED PORTIONS OF SAID CENTER TURNS; AND MEANS TO PREVENTDISPLACEMENT OF SAID TURNS WHEN SAID WINDING IS CONNECTED TO ANDDISCONNECTED FROM A SOURCE OF CURRENT.